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    给你一个由'1'（陆地）和'0'（水）组成的的二维网格，请你计算网格中岛屿的数量。
    岛屿总是被水包围，并且每座岛屿只能由水平方向和/或竖直方向上相邻的陆地连接形成。
    此外，你可以假设该网格的四条边均被水包围。


    示例 1：
    输入：grid = [
    ["1","1","1","1","0"],
    ["1","1","0","1","0"],
    ["1","1","0","0","0"],
    ["0","0","0","0","0"]
    ]
    输出：1

    示例 2：

    输入：grid = [
    ["1","1","0","0","0"],
    ["1","1","0","0","0"],
    ["0","0","1","0","0"],
    ["0","0","0","1","1"]
    ]
    输出：3

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